It is known to mount a fluid level sensor in the bottom of a fluid container for purposes of measuring the fluid level. Sensors of this type often include an ultrasonic transceiver that transmits ultrasonic pulses from the bottom of the container upwards through the fluid to the fluid surface. At the fluid surface, a portion of the ultrasonic pulses are downwardly reflected back to the transceiver. The transceiver receives the reflected pulses and in response, sends a corresponding signal to an electronic controller. The controller is able to calculate the depth of the fluid by knowing the roundtrip echo time of the ultrasonic signal and the approximate velocity of the ultrasonic signal in that particular fluid. However, the velocity of ultrasonic signals in fluids may be affected by a number of variables, including the composition and temperature of the fluid.
Thus, it is also known that the accuracy of the sensor described above may be enhanced by including a reference reflector at a known distance from the transceiver. In this arrangement, some of the ultrasonic signals are purposely directed to the reflector, instead of the fluid surface, such that they reflect off of the reflector and are transmitted back to the transceiver. The sensor again sends a signal to the electronic controller that is indicative of the roundtrip echo time of these reflected signals. Because the distance to the reference reflector is precisely known, the electronic controller is able to obtain an accurate signal velocity reading for the ultrasonic signal. This calibrated velocity reading may then be used with the echo time of the ultrasonic signals reflected from the fluid surface to obtain a more accurate fluid level reading. Alternatively, some devices compensate for signal velocity variations by including one or more temperature sensors positioned within the fluid container. The temperature sensors determine the fluid temperature and send corresponding signals to the controller, such that the controller may compensate for changes in speed based on known temperature vs. velocity characteristics of that fluid.
An example of such a fluid level sensor is shown in U.S. Pat. No. 5,226,320, issued Jul. 13, 1993 to Däges et al. This patent discloses a bottom-mounted fluid level sensor that includes a main sound pulse transceiver (S1) that transmits ultrasonic signals within a waveguide, such that the signals travel from the bottom of the tank to the fluid surface. Sound pulse transceiver S1 also transmits electronic signals to a controller, where the signals correspond to the roundtrip echo times of the ultrasonic signals. Additionally, the fluid level sensor of Däges et al. utilizes first and second reference sound pulse transceivers (S2, S3) for obtaining an accurate signal velocity reading and for measuring the distance to the bottom of the tank, respectively. The first reference sound pulse transceiver S2 transmits an ultrasonic reference signal along a reference path of known length such that it reflects off of a fixed reflecting surface and back to the transceiver. Transceiver S2 then sends a signal to the electronic controller indicating the roundtrip echo time of the ultrasonic signals reflecting from the fixed reference surface such that an accurate signal velocity reading may be determined. Transceiver S3, on the other hand, reflects ultrasonic signals off of the floor of the container to determine the level of the sump boundary; that is, the level of contaminants such as water and dirt that have settled on the bottom of the tank. This fluid level sensor also includes several temperature sensors positioned at various heights throughout the container that are used to determine a mean fluid temperature. The electronic controller is programmed to calculate a temperature-corrected sound velocity based on signals from both the reference transceiver and the temperature sensors, and in turn, derives a temperature-corrected fluid height measurement. However, a fluid level sensor constructed according to the Däges et al. patent is unable to perform both transmission and reception functions for both reference and measurement signals using a single transceiver.
Other examples of fluid level sensors using ultrasonic sensors and reference reflectors are shown in the following U.S. Pat. Nos.: 6,427,532 issued Aug. 6, 2002 to Keller, U.S. Pat. No. 5,856,953 issued Jan. 5, 1999 to Durkee, U.S. Pat. No. 5,309,763 issued May 10, 1994 to Sinclair, U.S. Pat. No. 5,121,340 issued Jun. 9, 1992 to Campbell et al., U.S. Pat. No. 5,095,748 issued Mar. 17, 1992 to Gregory et al. and U.S. Pat. No. 2,753,542 issued Jul. 3, 1956 to Rod et al, to name but a few.
Though the fluid level sensors taught in the above-mentioned patents are often times capable of providing sufficient fluid level readings, there still exists room for improvement. For instance, decreasing the size and cost of the sensors, increasing their accuracy, particularly in instances where the fuel is at a very low level, where the fuel is sloshing, or where the conditions are such that it makes it difficult for measurement, are just some of the areas that present continuing challenges.